1. The Field of the Invention
The present invention relates to the field of optoelectronic modules, and more particularly, to minimizing the amount of power consumed by an optoelectronic module using Thermo-Electric Coolers (TECs) for temperature control.
2. The Relevant Technology
Fiber optic technologies are increasingly used for transmitting voice and data signals. As a transmission medium, fiber optics provides a number of advantages over traditional electrical communication techniques. For example, light signals allow for extremely high transmission rates and very high bandwidth capabilities. Also, light signals are resistant to electromagnetic interferences that would otherwise interfere with and possibly degrade electrical signals. Light signals also can be transmitted over greater distances without the signal loss typically associated with electrical signals on copper wire.
While optical communications provide a number of advantages, the use of light as a transmission medium presents a number of implementation challenges. In particular, data carried by a light signal must be converted to an electrical format when received by a device, such as a network switch. Conversely, when data is transmitted to the optical network, it must be converted from an electronic signal to a light signal. The transmission of optical signals is typically accomplished by using a photonic device, such as a transceiver module, at both ends of a fiber optic cable. Each transceiver module typically contains a laser transmitter circuit capable of converting electrical signals to optical signals, and an optical receiver capable of converting received optical signals back into electrical signals.
These transceiver modules electrically interface with a host device, such as a host computer, switching hub, network router, switch box, computer input/output (I/O), or the like, via a compatible connection port. In some applications, it is desirable to miniaturize the physical size of the transceiver module to increase the number of transceiver modules that interface with the host device. By increasing the number of connection ports, the host devices accommodate a higher number of network connections within a given physical space. In some circumstances, it may be desirable for the transceiver module to be hot-pluggable, i.e., permitting inserting and removing of the transceiver module from a host device without interrupting electrical power.
To accomplish many of these objectives, and to ensure compatibility between different manufacturers, adopted international and industry standards define the physical size and shape of optical transceiver modules. For example, a group of optical component manufacturers developed a set of standards for optical transceiver modules termed Small Form-factor Pluggable (SFP) transceivers. In addition to the details of the electrical interface, this standard defines the physical size and shape for the SFP transceiver modules, and the corresponding connection port or module cage associated with the host device. These standards ensure interoperability between different manufacturers' products. More recently, the 10 Gb/s Small Form Factor Pluggable (XFP) standard was adopted, with all of the corresponding details concerning size, current draw, etc.
With smaller transceiver packages that meet the SFP or XFP standard, and increasing data rates, heat generated by the transceivers has become a problem. Heat dissipation mechanisms or cooling mechanisms alleviate the excessive heat created by the lasers and laser diodes within these transceivers. For instance, 10-Gigabit transceivers generally require heat dissipation mechanisms to operate in a standard temperature range of 15–30 degrees Celsius (° C.), whereas transceivers used with lower speed optical transmissions may not require heat dissipation. The use of heat dissipation mechanisms, however, increases the complexity and cost of assembling the transceiver, reduces the space that would otherwise be available for the functional optical and electrical components of the transceiver, and increases the amount of power required to operate the transceiver.
One type of heat dissipation or cooling mechanism is a thermoelectric cooler (TEC). A TEC maintains the temperature of a transceiver or a specific component of the transceiver at a predefined point. If the component gets too hot, current flows in one direction in the TEC to produce cooling. If the component gets too cold, the current flows in the other direction and the TEC acts as a heater. Unfortunately, TECs require much more power during cooling mode than heating mode. As the temperature of the module increases, the power consumed by the TEC for cooling increases exponentially.
FIG. 1 shows a graph 10 that is a schematic representation of the relationship between the current drawn by a TEC, shown as reference numeral 12, versus the difference in temperature between the hot and cold sides of the TEC (ΔT of the TEC). The zero power temperature for the TEC identified as “0”, i.e., the point where there is no difference in temperature between the cold and hot side of the TEC, shown as reference numeral 14. As can be seen from a plot 16 on graph 10, as the ΔT of the TEC becomes negative, the amount of current drawn by the TEC for heating goes up only slightly. This is because the TEC begins operating in heating mode, and draws a small amount of current. However, as the ΔT becomes positive, the amount of current drawn by the TEC for cooling goes up very rapidly. This is true because TECs are much more efficient heaters than coolers.
In a transceiver application, it is common for the base of the TEC (hot-side) to be thermally attached to the case of the transceiver, and for the laser to be attached to the top of the TEC (cold-side). For this common arrangement, the TEC is in a heating mode when the transceiver case temperature is lower than the desired laser set temperature, and conversely, the TEC is in cooling mode when the transceiver case temperature is higher than the laser set temperature. To operate the TEC as efficiently as possible, it is desirable to operate the TEC in a heating mode over a wide range of transceiver case temperatures.
With current transceiver modules designed to operate the laser in a temperature range from about 15° C. to about 30° C., a problem arises. Due to the working environment of typical transceiver modules, i.e., many modules mounted closely together, the operating case temperatures of the transceivers far exceed the desired laser temperature range, requiring that the TEC controlling the laser temperature run in the less efficient cooling mode. This presents a problem, because the overall amount of power available to the transceiver module, including the TEC, is limited, and expending a large amount of power to keep the transceiver's laser cooled to no more than 30° C. is undesirable.
A related problem is the bandwidth available to the transceiver module in the given temperature range. It is known in the art that the channel spacing can be tuned by adjusting the temperature. For instance, with a channel spacing of about 100 GHz, a temperature shift of about 10° C. is required to move between channels. For transceivers designed to operate at several DWDM channels, it is necessary to increase the temperature control range of the TEC to provide the necessary thermal tuning. To operate at a single 100 GHz channel, a temperature control range of approximately 10° C. is typically required. However, to operate over two 100 GHz channels, a temperature control range of approximately 20° C. is required. For multi-channel applications, it becomes even more critical to optimize the laser temperature range to minimize TEC power consumption.